1. Field of the Invention
This disclosure relates to a method for rendering an outline of a planar polygon when a map is displayed in a navigation device, for example, so that an edge line of the polygon is emphatically displayed or displayed in an emphatic manner, and an apparatus employing the method.
2. Description of the Related Art
In a navigation device, for example, map data corresponding to a map including a current position or a specified point as a center is read from a database in a map/information data recording medium such as a DVD (Digital Versatile Disc) or a hard disk so that the map is displayed. In many cases, the map data in the database recorded in the map/information data recording medium is divided as map meshes having an appropriate size of a longitude width and a latitude width in accordance with various scale levels, and roads and the like are stored as collections of coordinates of nodes represented using the longitude and the latitude. Each of the roads is constituted by a connection of at least two nodes. The map data includes road data including a road list, a node table, and a list of nodes constituting intersection, polygon data used to display the roads, architectures, facilities, parks, and rivers, and information data used to display map symbols and text representing names of administrative areas, names of the roads, names of the intersections, and names of the facilities, for example.
Various examples of the polygon data include a polygon line. As shown in examples of maps in FIGS. 13A and 13B, facilities having planer shapes to be emphasized are displayed as polygons and roads and railway tracks in which positions of their lines rather than shapes are to be emphasized are displayed as polylines.
For example, in the maps shown in FIGS. 13A and 13B which are displayed in a monitor, facilities are displayed as polygons and roads and railway tracks are displayed as polylines. In such map display, if a user wishes to obtain a shape of a specific facility among the facilities denoted by the various polygons, the user can easily recognize the shape when a technique of emphasizing an edge line of a polygon corresponding to the specific facility so that the shape of the polygon is clearly displayed is used. According to this technique, as shown in FIG. 13B, when a cursor is moved to point a specific facility, outlines are displayed by heavy lines so that the specific facility is clearly displayed. Note that in addition to the technique of displaying an outline of a specific one of various polygons included in a map by a heavy line, a technique of displaying outlines of all polygons in an emphatic manner with heavy lines may be used.
Note that Japanese Unexamined Patent Application Publication No. 2004-132814 discloses a technique of correcting boundary lines of a polygon in accordance with a region in an extracted range so that the polygon is redefined and delivering road map data. Furthermore, Japanese Unexamined Patent Application Publication No. 11-191111 discloses a technique of extracting a first small polygon including an arbitrary point to be included in a polygon and further extracting a second small polygon adjacent to the first small polygon so that the polygon is constituted by small polygons and generating the polygon by connecting the small polygons to one another.
As described above, especially when a large national park or a large airport is to be viewed in a wide-area map in which an outline of a polygon is displayed in an emphatic manner, the polygon may be included in a plurality of map meshes. For example, when a large polygon denoted by hatching as shown in FIG. 9A is displayed, data of the polygon may be included in map meshes 1 to 18. This polygon data is displayed so that boundary line portions of the map meshes are inconspicuously displayed as shown in FIG. 9B.
When the polygon displayed as shown in FIG. 9B is instructed to be displayed in an emphatic manner so as to be clearly viewed, a map display processor displays the outline of the polygon included in the map meshes in an emphatic manner as shown in FIG. 9C and combines the map meshes with one another so that the polygon is displayed as shown in FIG. 9D. Accordingly, the polygon is not clearly viewed or at least appearance of the polygon is not good. Examples of such display include display of a map of the area around Great Lakes shown in FIG. 7A and display of an airport shown in FIG. 8A.
In such polygon emphasis display as described above, when an outline of a polygon is displayed by a heavy line, map mesh boundary lines are also emphasized. To address this problem in which an entire map is not clearly viewed, a process of not displaying map mesh boundary line portions as shown in FIG. 9D, for example, may be used as a solution. This process is effective to the case where, as shown in FIGS. 9A to 9D, the entire outline of the polygon is a curved line and is not included in any of the boundary lines. However, in a case where a portion of the outline of the polygon overlaps with one of the map mesh boundary lines, when the process of not displaying map mesh boundary line portions is performed, the portion of the outline of the polygon may be removed. Examples of this case are shown in FIGS. 10A and 10B to FIGS. 12A and 12B.
For convenience of description, in FIGS. 10A and 10B to FIGS. 12A and 12B, an outline of a polygon is constituted by lines a to b, b to c, c to d, . . . , and q to a as examples. In the examples shown in FIGS. 10A and 10B to FIGS. 12A and 12B, in the outline, the line d to e overlaps with a horizontal boundary line of map meshes, the line e to f overlaps with a vertical boundary line of map meshes, the line i to j overlaps with a vertical boundary line, the line l to m overlaps with a horizontal boundary line, the line p to q overlaps with a horizontal boundary line, and the line q to a overlaps with a vertical boundary line.
As described above, when the outlines overlap with the map mesh boundary lines, data blocks of the polygon are independently included in the respective map meshes as shown in FIG. 10B. Therefore, in FIG. 11A in which polygon data shown in FIG. 10A is displayed, when the user provides instruction to display the polygon in an emphatic manner by pointing the polygon by a cursor, since the outline of the polygon is emphatically displayed in the individual map meshes as shown in FIG. 11B, the map meshes boundary line portions of the polygon are also displayed in the individual map meshes in an emphatic manner. Accordingly, the polygon to be displayed by combining the map meshes includes the map mesh boundary lines which are displayed in an emphatic manner as shown in FIG. 12A. Consequently, as with the case of FIG. 9D, the polygon is not clearly viewed, and at least appearance of the polygon is not good.
To address this problem, if lines on the map mesh boundary lines are removed when the polygon as shown in FIG. 12A is displayed, portions of the outline of the polygon which are positioned on the map mesh boundary lines are also removed as shown in FIG. 12B. Accordingly, also in this case, the polygon is not clearly viewed, and at least appearance of the polygon is not good. That is, when the outline of the polygon is displayed in an emphatic manner, the map mesh boundary line portions are also emphatically displayed whereas when the map mesh boundary lines are merely removed, the portions of the outline of the polygon on the map mesh boundary lines are also removed, and accordingly, this is not an efficient solution.